In manufacturing semiconductor devices, various thermal processes such as an oxidation process or a CVD process (chemical vapor deposition) are conducted to semiconductor wafers. As a thermal processing unit of a batch type that carries out such a thermal process, a longitudinal type of thermal processing unit is known. In the unit, a large number of wafers are held by a holder, which is called a wafer boat, in a tier-like manner, and the holder is conveyed into a longitudinal type of thermal processing furnace, for example thorough a lower part of the thermal processing furnace. Then, atmosphere in the thermal processing furnace is made to be heating atmosphere of a predetermined temperature, and a thermal process is conducted. In general, area of the thermal processing furnace to be heated is vertically divided into a plurality of zones, and a heating unit and a temperature controller are provided for each zone in order to enable a temperature control for each zone.
FIG. 10 is a longitudinal sectional view of an oxidation processing unit as an example of a longitudinal thermal processing unit. In the drawing, 11 represents a reaction container whose lower end is open. Gas-supplying pipes 12 and 13 for supplying a or more gases for the oxidation process, such as an oxygen gas and a hydrogen-chloride gas, into the reaction container 11, and a gas-discharging pipe 15 that extends to a gas-discharging pump 14, are connected to a lower end portion of the reaction container 11. The gas-supplying pipes 12 and 13 are bent upward in the reaction container 11, and are opened in the vicinity of a ceiling part of the reaction container 11 to supply the respective gases therefrom. In the drawing, 16 represents a plurality of vertically-divided heaters provided around the reaction container 11. 17 represents a wafer boat for holding a large number of wafers W in a tier-like manner.
A flow of an oxidation process using the apparatus shown in FIG. 10 is briefly explained as follows. At first, for example 140 wafers W having surfaces on which silicon layers have been formed are conveyed onto the wafer boat 17. Then, the wafer boat 17 is conveyed into the reaction container 11. At that time, a lower-end opening 19 of the reaction container 11 is hermetically sealed by a lid 18 provided at a lower end of the wafer boat 17. Then, the inside of the reaction container 11 is heated to a predetermined temperature, and maintained in a state of a slightly-reduced pressure. Under the state, the process gas or gases, such as an oxygen gas and a hydrogen-chloride gas, are introduced into the reaction container 11, and thus surfaces of the wafers W are oxidized to form silicon oxide films.
When the above oxidation process is conducted, in order to make thickness of the silicon oxide films uniform between the wafers, an adjusting operation of temperature set values of the respective heaters 16 is carried out in advance (for example, claim 1 and paragraph 0003 of JP-A-2001-77041). Then, by means of a temperature controlling part not shown, a temperature control for each heater 16 is carried out in such a manner that the respective heaters 16 are maintained at the above temperature set values.
In the above adjusting operation of temperature set values, for example, dummy wafers having surfaces on which silicon oxide films have been formed are held by the wafer boat 17 in accordance with a layout depending on an arrangement layout of production wafers to be processed. Then, the thermal process (oxidation process) is conducted in accordance with predetermined temperature set values in such a manner that thickness of obtained (formed) oxide films substantially coincides with a target thickness. Then, thickness of the silicon oxide films is measured for each zone of the thermal processing atmosphere, a calculation is conducted based on a relationship between variation of the film thickness and variation of the temperature set values of the respective heaters 16, which has been obtained in advance, and then the temperature set values are corrected.
If thickness profile is not made flat by one correction, the above correction (adjusting operation) is repeatedly conducted until the thickness profile is maintained within an error range of the target thickness.
Herein, the inventor has studied a method of: creating reduced-pressure atmosphere in a reaction container, generating active species including O-radical and OH-radical by introducing a hydrogen gas and an oxygen gas into the reaction container at predetermined flow rates and by heating them to for example about 1000° C., and forming oxide films onto wafers by means of the radicals. According to the method, oxide films having good film quality can be obtained because an oxidative effect of the method is stronger than those of dry oxidation process and wet oxidation process.
On the other hand, the above document reveals that production wafers are used for the above adjusting operation. However, the temperature adjusting operation includes a plurality of thermal processes. Thus, if the production wafers are used, especially if the production wafers have larger diameters, burden of cost is great, which is not preferable. Therefore, in general, dummy wafers are used for the temperature adjusting operation.
However, in the above oxidation process using the radicals, even if the adjusting operation of the temperature set values is conducted by using dummy wafers, uniformity of thickness of oxide films formed on production wafers between surfaces (between wafers) is bad. In particular, there is tendency (loading effect) that film thickness of wafers located at a portion on a downstream side of the process gas is thinner than that of wafers located at a portion on an upstream side thereof.
FIG. 11 shows a result of an experiment for confirming the loading effect. The temperature set values of the respective heaters 16 for the respective zones were adjusted to values according to which uniformity of film thickness between surfaces is expected to be satisfactory. Then, wafers W are fully arranged onto the wafer boat 17. An oxidation process by radicals was conducted under the same condition for the following three cases: a case wherein all the wafers W were dummy wafers; a case wherein 22 wafers (trench wafers) whose surfaces have trench structures were arranged, and a case wherein 39 trench wafers were arranged. Film thickness profile was measured for each case. ♦ is the result for the case wherein all the wafers W were dummy wafers. ▴ is the result for the case wherein 22 trench wafers were used. ▪ is the result for the case wherein 39 trench wafers were used.
As seen from the results, regarding the dummy wafers on which the enough thick oxide films had been formed, uniformity of film thickness between surfaces is not so bad, although there was tendency that film thickness of wafers located at a portion on a downstream side is thin to some extent. However, when the trench wafers were used, the tendency that film thickness of wafers located at a portion on a downstream side is thin is strong. The tendency is stronger as the number of trench wafers is increased more.
The inventor thinks about the cause as follows. That is, as the wafers (trench wafers) having deep patterns (trench) have large surface areas, consumption amount of the radicals is thought to be large. On the other hand, as the dummy wafers have thick oxide films on flat surfaces thereof having no patterns, consumption amount of the radicals on surfaces of the oxide films is thought to be less than on the trench wafers.
On the other hand, if the flow rate of the process gas is increased, uniformity of film thickness within a surface of each substrate may be decreased. In addition, the pressure of the reaction chamber 11 is increased, and hence the radicals may easily lose activity, that is, lifetime of the radicals may be shortened. Thus, it is not preferable to set a large flow rate. Thus, the oxidation process using the radicals may suffer from the loading effect in the conventional manner, so that it is difficult to secure high uniformity of film quality of the oxide films between surfaces.
In addition, if process condition such as a temperature is determined depending on the arrangement layout of the production wafers, the setting operation of the process condition is troublesome. In addition, if process condition such as a temperature of each zone of the reaction container, a pressure, a gas flow rate and a processing time, is set depending on the number and/or the arrangement layout of production wafers arranged on the wafer boat, some operation miss is likely to occur.